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Optimizing AS Paths


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In part 4 of this BGP webinar series, we cover how to optimize Autonomous System paths, especially in cases of suboptimal international routes. Tips include monitoring layer 3 forwarding alongside BGP, monitoring reverse paths, layering covering prefixes, as well as prepending and MED.

Starting from key concepts, you'll learn how to recognize route leaks and hijacks in the data, alert for these events and proactively mitigate their impact. See the webinar recording at

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Optimizing AS Paths

  1. 1. Archana Kesavan, Product Marketing Manager Optimizing BGP AS Paths
  2. 2. About ThousandEyes Network Intelligence platform that gives you a complete picture from users to internal and cloud- based applications Surface insights from a global data set Lightweight, flexible data collection Unified view of diverse performance data Solve issues across shared infrastructure See any network like it’s your own
  3. 3. 2 • Generally prefers the shortest AS Path • Generally trusted advertisements • Has quick convergence across the Internet • Follows the route of the most specific prefix BGP Strengths and Limitations • It’s often not the most performant • Hijacks and leaks, even from trusted sources • Flapping and route instability • Many covered prefixes not finely tuned BGP… Yet…
  4. 4. 3 When BGP Delivers Suboptimal Results • Developing country connections • Peering at remote IXs • Underlying capacity issues Examples Two sources in Taiwan, one target in Taiwan, one path via the U.S. (80-150ms)
  5. 5. 4 Common, Perhaps Suboptimal, Peering Points Miami Los Angeles San Jose FrankfurtLondon Amsterdam
  6. 6. 5 1. Monitor the layer 3 path 2. Monitor both forward and reverse paths 3. Evaluate your peering policies 4. Layer covering and covered prefixes 5. Prepending, MED and advertisements 6. Consider Anycast where appropriate Optimizing AS Paths
  7. 7. 6 • Map actual traffic paths to BGP routes • Understand loss and latency of the path • Does the path double- back across the Pacific? Peer in Frankfurt? Monitor the Layer 3 Path Optimizing BGP #1: Intra-Asia traffic peering in LAX
  8. 8. 7 • See how asymmetric routing impacts performance • Decompose loss and latency in each direction • Agent-to-Agent tests for reverse path; Private BGP Monitors for reverse routes Monitor Both Forward & Reverse Path Optimizing BGP #2: EWRàLON via Zayo EWRßLON via Telia DFWàLON via Telia DFWßLON via Softlayer
  9. 9. 8 • Reduce suboptimal routing by increasing peering • Consider primary ISPs with most/nearest peering connections • Consider peering with a regional IXP How Well Do You Peer? Optimizing BGP #3: Google’s peering
  10. 10. 9 • Use prefixes to your advantage • Place backup policies/routes in a covering prefix • Especially for DDoS mitigation, failover routes Covered & Covering Prefixes Optimizing BGP #4:
  11. 11. 10 • Prepending can make a route less desirable, but can have unintended consequences • MED can signal which routes your prefer to be be propagated • Communities can be used to coordinate route propagation with your ISP Prepending, MED and Communities Optimizing BGP #5:
  12. 12. 11 • For some services (even TCP!) Anycast can make sense • Broadcast routes for the same prefix from multiple origins • Make sure to validate optimal routes Consider Anycast Optimizing BGP #6: J Root: 6 origin Autonomous Systems
  13. 13. 12 Demo
  14. 14. 13 Demo 1: Suboptimal BGP Routing between UK and China hosted in China At 9:55 UTC, Traffic from Manchester to traverses the Sprint network in US
  15. 15. 14 Demo 1: Suboptimal BGP Routing between UK and China BGP Route Visualization for the covered prefix 123. 125.65.0/18 Validate BGP routes for both covered and covering prefixes
  16. 16. 15 Demo 1: Suboptimal BGP Routing between UK and China DNS round robin resolves to a different IP address in every iteration For the new destination IP address, traffic path and BGP routing is optimized. At 9:50 UTC, traffic from Manchester to (hosted in Beijing) takes a shorter path and does NOT traverse the US
  17. 17. 16 Demo 2: Prepending error with BGP Route Change Path Visualization indicates a single upstream ISP “Access2Go” for Country Financial before the outage. Connectivity issues indicated by a dip in availability and increased packet loss to Country Financial
  18. 18. 17 Demo 2: Prepending error with BGP Route Change BGP Route Visualization confirms that AS 40948 (Access2Go) is the only upstream providers for the origin network, AS AS 10511(Country Financial)
  19. 19. 18 Demo 2: Prepending error with BGP Route Change After the outage, Country Financial peers with Qwest Communications and not Access2Go.
  20. 20. 19 Demo 2: Prepending error with BGP Route Change BGP Path Visualization confirms AS 209 (Quest) peering with origin AS 10511 (Country Financial) But who is AS 15011? And no monitors peer with AS 15011
  21. 21. 20 Demo 2: Prepending error with BGP Route Change AS 15011 belongs to Jaguar Communication and was mistakenly included in the BGP AS Path while using path prepending to increase the cost of the BGP path .
  22. 22. 21 See what you’re missing. Watch the webinar